[SI-LIST] Re: Effects of steam?
- From: "Stefan Milnor" <stefan.milnor@xxxxxxxxxxxxxx>
- To: "Yuriy Shlepnev" <shlepnev@xxxxxxxxxxxxx>, "Howard Johnson" <howie03@xxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Mon, 24 Nov 2008 15:19:29 -0800
Sounds like a case for conformal coating.
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Yuriy Shlepnev
Sent: Monday, November 24, 2008 2:58 PM
To: 'Howard Johnson'; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Effects of steam?
Howard,
Your estimation of the water properties may be correct in 4-5 GHz range.
Dielectric properties of water can be approximated with one-pole Debye model as
follows:
E(w)=Einf+(Es-Einf)/(1+j*w*tr)
Einf - value at infinity, may be between 1.8 [1] and 23 [2] Es - value at DC,
may be between 81 [1] and 82 [2] tr - relaxation time, may be between 9.4 ps
[1] and 11 ps [2]
1) K.S. Kuntz, R.J. Luebers, The finite difference time domain method for
electromagnetics, page 124
2) E.G. Farr, C.A. Frost, Impulse Propagation Measurements of the Dielectric
Properties of Water, Dry Sand, Moist Sand, and Concrete
http://www.farr-research.com/Papers/mn52.pdf, page 7
At 1 GHz it gives DK=80 and LT=0.046 according to [1] and DK=80 and LT=0.063
according to [2].
At 10 GHz, DK=63 and LT=0.42 according to [1] and DK=58 and LT=0.63 according
to [2] I used tr=9.4 ps in both cases - the difference arises due to the
different values at infinity.
Loss tangent is about 0.2 and DK=76 at 4.9 GHz according to data from [1].
Best regards,
Yuriy Shlepnev
www.simberian.com
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Howard Johnson
Sent: Monday, November 24, 2008 1:30 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Effects of steam?
Dear John Barrett,
I don't have exact numbers, but I think the problem will be much worse that
you imagine. Here is my reasoning.
(1) Water can (apparently) induce some types of lamination failure. To address
that problem, board manufacturers long ago agreed to a standard delamination
test for water absorbtion. In this test, if I remember it correctly, the
finished pcb is subjected to a hot, high humidity environment. Then you try to
pull the board apart (de-laminating it) with suction cups. Anyway, after
processing, the board can also be weighed to determine the percentage water
absorption. That's what you want to know. I remember figures like 1% being
rated as "OK" for pcb laminates (please check the number with your board
supplier).
(2) I do not know the density of your board material, but if I did, I would
take the percentage water absorption by weight and from it compute the
percentage water BY VOLUME, which is what counts. Call that percentage "alpha".
(3) The complex dielectric permittivity of your wet board may be approximated
by the following equation. This equation takes into account both the real and
imaginary parts of permittivity.
E(wet) = (1-alpha)E[polyimide] + (alpha)E[water]
(4) Now we get to the interesting part. The complex dielectric permittivity of
water has a magnitude of about 80 (eighty), and a terrible loss tangent (don't
recall how bad -- but this is part of the reason microwave ovens work so well).
Since the magnitude of E[water] is so huge (80), it doesn't take much of an
"alpha" to significantly degrade the properties of your laminate.
For example, (please don't quote these numbers this is just an example):
Park Nelco N7000 polyimide resin/e-glass: E[polyimide] = 3.8*(1 - 0.016j)
Water: E[water] = 80*(1 - 0.2j) (just a guess for this example)
Alpha=0.01 (one-percent absorbtion)
Result: E[wet] = 0.99*3.8*(1 - 0.016j) + 0.01*80*(1 - 0.2j)
= 3.762 - 0.0608j + 0.80 - 0.16j
= 4.562 - .2208
The loss tan of the result is .2208/4.562 = 0.048, triple the original figure.
The water component, even if it doesn't change the overall dielectric constant
very much, can change the loss tangent a lot.
RELATED ISSUE: People who make low-loss capacitors keep their materials dry.
That seems closely related to your issue. The following designers guide for
capacitors says, "Water in printed circuit laminates is responsible for a form
of dielectric absorption called "hook" that causes many problems for users of
high impedance and high frequency attenuators."
http://www.designers-guide.org/Modeling/da.pdf
I hope these brief comments are helpful to you.
If anyone has the dielectric loss numbers for water, I'd like to see them,
please.
Best regards,
Dr. Howard Johnson, Signal Consulting Inc., tel +1 509-997-0505,
howie03@xxxxxxxxxx www.sigcon.com -- High-Speed Digital Design seminars,
publications and films
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of John Barrett
Sent: Monday, November 24, 2008 4:09 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Effects of steam?
This one is a bit off the beaten track but I want to operate an organic circuit
board (no devices, just gold conductors on polyimide), in an autoclave with
high pressure (3 bar), high temperature (135C) non-saturated steam. There's no
condensation and so no real water about. What would be the effect of steam
absorption on the dielectric constant and the loss tangent up to 5GHz?
Non-saturated steam, i.e. dry, invisible steam, has dielectric properties very
close to air so, if it were to penetrate a porous material e.g. porous alumina,
then it would just displace the air in the pores and cause little variation in
electrical parameters. With something like polyimide, which I presume absorbs
moisture inter-molecularly, there are measureable shifts in dielectric
properties with variations in moisture content caused by humidity variations at
normal temperatures, and I think that this is caused by the release of extra
charges, but what are the effects in 135C, 3bar steam? I could measure the
effects with resonator test structures and a VNA but I would need to find and
invest in high frequency connectors and cables able to withstand the autoclave
atmosphere - not cheap, I imagine, even if they exist. Perhaps there are
dielectric experts on the list who might be able to speculate from a materials
science viewpoint?
Regards and thanks,
John
Séan de Baróid
Ionad um Raidichórais Inoiriúnaithe
Roinn na hInnealtóireachta Leictreonaí
Institiúd Teicneolaíochta Chorcaí
________________
John Barrett
Centre for Adaptive Wireless Systems
Department of Electronic Engineering
Cork Institute of Technology
"Beidh fáilte roimh freagra as Gaeilge"
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